In a conventional rack-and-pinion steering system for motor vehicles and the like, a rack bar has been employed. Recently, it has been tried to hollow the rack bar so as to make it more lightweight. In a method of manufacturing such a hollow rack bar, as disclosed in Publication of Examined Japanese Patent Application No. Hei 3-5892, an electric resistance welded tube or an extrusion molded steel pipe is employed and a flat portion is formed in the periphery of the tube or the steel pipe by press working. In the flat portion are formed rack teeth by placing the steel pipe having the flat portion in a tooth profile forming die-and forcibly inserting a mandrel into the steel pipe, thereby extruding the flat portion from the inside.
However, such a conventional method has a problem that the rack teeth configuration of the tooth profile forming die, particularly the central portion of the rack teeth configuration, is continuously subjected to an excessive load. Also, transverse end portions of the rack teeth cannot be formed precisely. Furthermore, resistance by processing imposed on the mandrel is significant.